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  <div class="section" id="module-copy">
<span id="copy-shallow-and-deep-copy-operations"></span><h1><a class="reference internal" href="#module-copy" title="copy: Shallow and deep copy operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copy</span></code></a> --- 浅层 (shallow) 和深层 (deep) 复制操作<a class="headerlink" href="#module-copy" title="永久链接至标题">¶</a></h1>
<p><strong>源代码:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.7/Lib/copy.py">Lib/copy.py</a></p>
<hr class="docutils" />
<p>Python 中赋值语句不复制对象，而是在目标和对象之间创建绑定 (bindings) 关系。对于自身可变或者包含可变项的集合对象，开发者有时会需要生成其副本用于改变操作，进而避免改变原对象。本模块提供了通用的浅层复制和深层复制操作（如下所述）。</p>
<p>接口摘要：</p>
<dl class="function">
<dt id="copy.copy">
<code class="descclassname">copy.</code><code class="descname">copy</code><span class="sig-paren">(</span><em>x</em><span class="sig-paren">)</span><a class="headerlink" href="#copy.copy" title="永久链接至目标">¶</a></dt>
<dd><p>返回 <em>x</em> 的浅层复制。</p>
</dd></dl>

<dl class="function">
<dt id="copy.deepcopy">
<code class="descclassname">copy.</code><code class="descname">deepcopy</code><span class="sig-paren">(</span><em>x</em><span class="optional">[</span>, <em>memo</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#copy.deepcopy" title="永久链接至目标">¶</a></dt>
<dd><p>返回 <em>x</em> 的深层复制。</p>
</dd></dl>

<dl class="exception">
<dt id="copy.error">
<em class="property">exception </em><code class="descclassname">copy.</code><code class="descname">error</code><a class="headerlink" href="#copy.error" title="永久链接至目标">¶</a></dt>
<dd><p>针对模块特定错误引发。</p>
</dd></dl>

<p>浅层复制和深层复制之间的区别仅与复合对象 (即包含其他对象的对象，如列表或类的实例) 相关:</p>
<ul class="simple">
<li>一个 <em>浅层复制</em> 会构造一个新的复合对象，然后（在可能的范围内）将原对象中找到的 <em>引用</em> 插入其中。</li>
<li>一个 <em>深层复制</em> 会构造一个新的复合对象，然后递归地将原始对象中所找到的对象的 <em>副本</em> 插入。</li>
</ul>
<p>深度复制操作通常存在两个问题, 而浅层复制操作并不存在这些问题：</p>
<ul class="simple">
<li>递归对象 (直接或间接包含对自身引用的复合对象) 可能会导致递归循环。</li>
<li>由于深层复制会复制所有内容，因此可能会过多复制（例如本应该在副本之间共享的数据）。</li>
</ul>
<p>The <a class="reference internal" href="#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate"><span class="pre">deepcopy()</span></code></a> function avoids these problems by:</p>
<ul class="simple">
<li>保留在当前复制过程中已复制的对象的 &quot;备忘录&quot; （<code class="docutils literal notranslate"><span class="pre">memo</span></code>） 字典；以及</li>
<li>允许用户定义的类重载复制操作或复制的组件集合。</li>
</ul>
<p>该模块不复制模块、方法、栈追踪（stack trace）、栈帧（stack frame）、文件、套接字、窗口、数组以及任何类似的类型。它通过不改变地返回原始对象来（浅层或深层地）“复制”函数和类；这与 <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> 模块处理这类问题的方式是相似的。</p>
<p>制作字典的浅层复制可以使用 <a class="reference internal" href="stdtypes.html#dict.copy" title="dict.copy"><code class="xref py py-meth docutils literal notranslate"><span class="pre">dict.copy()</span></code></a> 方法，而制作列表的浅层复制可以通过赋值整个列表的切片完成，例如，<code class="docutils literal notranslate"><span class="pre">copied_list</span> <span class="pre">=</span> <span class="pre">original_list[:]</span></code>。</p>
<p id="index-0">类可以使用与控制序列化（pickling）操作相同的接口来控制复制操作，关于这些方法的描述信息请参考 <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> 模块。实际上，<a class="reference internal" href="#module-copy" title="copy: Shallow and deep copy operations."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copy</span></code></a> 模块使用的正是从 <a class="reference internal" href="copyreg.html#module-copyreg" title="copyreg: Register pickle support functions."><code class="xref py py-mod docutils literal notranslate"><span class="pre">copyreg</span></code></a> 模块中注册的 pickle 函数。</p>
<p id="index-1">想要给一个类定义它自己的拷贝操作实现，可以通过定义特殊方法 <code class="xref py py-meth docutils literal notranslate"><span class="pre">__copy__()</span></code> 和 <code class="xref py py-meth docutils literal notranslate"><span class="pre">__deepcopy__()</span></code>。 调用前者以实现浅层拷贝操作，该方法不用传入额外参数。 调用后者以实现深层拷贝操作；它应传入一个参数即 <code class="docutils literal notranslate"><span class="pre">memo</span></code> 字典。 如果 <code class="xref py py-meth docutils literal notranslate"><span class="pre">__deepcopy__()</span></code> 实现需要创建一个组件的深层拷贝，它应当调用 <a class="reference internal" href="#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate"><span class="pre">deepcopy()</span></code></a> 函数并以该组件作为第一个参数，而将 memo 字典作为第二个参数。</p>
<div class="admonition seealso">
<p class="first admonition-title">参见</p>
<dl class="last docutils">
<dt>模块 <a class="reference internal" href="pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a></dt>
<dd>讨论了支持对象状态检索和恢复的特殊方法。</dd>
</dl>
</div>
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